Problem with loop for

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0 个投票

Hello

I need to calculate average value using various methods including loop for. I don't know why result using loop is diffrent. Can someone help with that?

N = 1200; 
A = 2; 
fx = 5; 
fp = 1000;  
dt = 1/fp;              
t = dt*(0:N-1);         
x = A*sin(2*pi*fx*t); 
x_sred1 = mean(x);   %average value
x_sred2 = sum(x)/N;
disp(['x_sred1 = ' ,string(x_sred1), 'x_sred2 = ' ,string(x_sred2)])
    "x_sred1 = "    "-1.3414e-16"    "x_sred2 = "    "-1.3414e-16"
%average value loop for
l=length(x);
s_x=0;
for i=1:l
    s_x=s_x+x(i);
end
x_sred3=s_x/N;
disp(['x_sred3 = ' ,string(x_sred3)])
    "x_sred3 = "    "9.142e-17"

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Mathieu NOE 2024-3-6

No , the code is correct in its principle, but your for loop demonstrate that this method is accumulating more errors than the two first methods

with the choosen parameters , your sum should be zero , but you see actually some difference (as tiny numbers) because of round off accumated errors.

now if you can make a small change in your parameters to get a sum that is not (almost) zero , then all 3 computations are very close.

here for example I simply change N from 1200 to 1150 and I get :

"x_sred1 = " "0.056223" "x_sred2 = " "0.056223"

"x_sred3 = " "0.056223"

or N = 1200 and fp = 5000 (to increase time accuracy)

"x_sred1 = " "0.1825" "x_sred2 = " "0.1825"

"x_sred3 = " "0.1825"

VBBV 2024-3-8

在 MATLAB Online 中打开

@Joanna Zajac, It is something to do with property of sin function for specific values in input array

N = 1200; 
A = 2; 
fx = 5; 
fp = 1000;  
dt = 1/fp;              
t = dt*(0:N-1);         
x = A*(2*pi*fx*t); 
x_sred1 = mean(x)   %average value
x_sred1 = 37.6677
x_sred2 = sum(x)/N
x_sred2 = 37.6677
disp(['x_sred1 = ' ,string(x_sred1), 'x_sred2 = ' ,string(x_sred2)])
    "x_sred1 = "    "37.6677"    "x_sred2 = "    "37.6677"
%average value loop for
% l=length(x);
s_x=0;
for i=1:numel(x)
    s_x=s_x+x(i);
end
x_sred3=s_x/N;
disp(['x_sred3 = ' ,string(x_sred3)])
    "x_sred3 = "    "37.6677"